We might want to sanitize or modify some data at the parent Graphql API before we delegate it to some underlying Graphql API. This can be achieved with the help of a custom resolver as illustrated below.

Let us consider a case where the insert_user mutation has to be delegated to an underlying schema. Before delegating, the email in the data has to be converted to all lowercase at the parent Graphql API level.

1. Create a remote executable schema with the makeRemoteExecutableSchema() function https://gist.github.com/aswinmprabhu/61829a0197e48f227651b33b64d1d8e2
2. Create a custom resolver for our user case https://gist.github.com/aswinmprabhu/6b3902ed6193cefcd4f0c9c3902d7c3b
3. Merge the schemas together with the mergeSchemas() function https://gist.github.com/aswinmprabhu/0d867878093b764de87c4a965f811f42
4. Serve the merged schema with a new instance of the apollo server https://gist.github.com/aswinmprabhu/7d7c863854558a88b981adfc5bac5ef0